Trigger frame event on NMI, with trigger on vcounter() == 241 as fallback

[Brunnis]

This is a fix that removes one full frame of input lag. This fix is identical to the one made for bsnes-mercury; see pull request with details here:

https://github.com/libretro/bsnes-mercury/pull/18

This pull request includes the changes proposed by Alcaro here: https://github.com/libretro/bsnes-mercury/pull/18#issuecomment-229416570

The implementation has been successfully tested in the same way as the bsnes-mercury one, i.e.:

• Confirm input lag improvement for all three profiles (accuracy, balanced, performance).
• Confirm that defining SFC_LAGFIX=0 results in original behavior for all three profiles.
• Analyze behavior of the last scanline with no overscan and overscan.

[Brunnis]

Some more testing with games using overscan reported here: http://libretro.com/forums/showthread.php?t=5428&p=41937&viewfull=1#post41937

No issues found.